A nucleic acid ligand (aptamer) is a nucleic acid macromolecule (e.g., DNA or RNA) that binds tightly to a specific molecular target. Like all nucleic acids, a particular nucleic acid ligand may be described by a linear sequence of nucleotides (A, U, T, C and G), typically 15-40 nucleotides long. In solution, the chain of nucleotides forms intramolecular interactions that fold the molecule into a complex three-dimensional shape. The shape of the nucleic acid ligand allows it to bind tightly against the surface of its target molecule, generally binding their targets with very high affinity, e.g., the majority of anti-protein nucleic acid ligands have equilibrium dissociation constants in the picomolar to low nanomolar range.
However, high affinity does not guarantee specificity. Even with high affinity aptamers, non-specific binding or cross-reactivity is a problem (Commercialization of an Aptamer-Based Diagnostic Test, 2012; Andy Ellington's Blog, 2011; Van Simaeys, 2010; Li, 2009; Rahimi, 2009).